The invention relates to the area of water treatment.
More precisely, the invention relates to a method to treat water (for its purification or potabilisation) comprising a flocculation-decantation step using floc ballasted by a granular material, according to which the ballast (granular material) that is used is at least partially recycled.
The invention also concerns the field of circular grit traps used for such recycling.
The so-called ballasted, physico-chemical flocculation-decantation technique consists of adding solid settling agents, or ballast, to the water to be treated, these being chiefly fine sand, a polymer or possibly other coagulation-flocculation agents, then of allowing floc formation around the sand. The floc, which without sand has a density very close to that of water, then settles quicker and makes it possible to use a smaller decantation surface. The sand is then recovered using mechanical separating means. The separated sludge is then directed towards a specific treatment, while the microsand and that part of the sludge which has not been separated by the sand recovery treatment, are re-added to the head of the process. Such technique is described in particular in French patent FR 1411792.
The mechanical separating means used to separate the sand from the sludge are most often made up of static centrifugal graders of cylindrical-conical shape called circular grit traps.
During said mechanical separating step, it is most frequently sought, by means of a circular grit trap, to recover sand having a particle size greater than a given particle size and to separate this sand from surrounding fine particles, so-called xe2x80x9cfinesxe2x80x9d.
In respect of water treatment, it is sought to recover the sand having a particle size greater than a given size at the downflow outlet, and the sludge at the upflow outlet. It may also be desired to wash the recycled sand of its surrounding sludge, for example to minimize the probability of recycling undesirable micro organisms at the head of a floc decanter ballasted with sand, or further to avoid, in the same type of equipment, an excessive increase in fines in the decanter, for example during water treatment of a river in spate.
The sludge may be so-called light sludge, having a density close to that of water, and then distributes itself between the upflow and downflow outlets of a usual circular grit trap in the same proportions as water.
It may also be so-called heavy sludge, clay for example and mineral particles, whose density is close to that of sand, and it therefore tends to be directed towards the downflow outletxe2x80x94and therefore to be recirculated with the sand without serving any helpful purpose and possibly even being harmful to the proper functioning of the methods applied.
It is to be noted that circular grit traps do not provide a clean-cut separation in respect of the given particle size. For each particle size the probability of capturing this size is only average. The efficacy of a grit trap depends upon the density and diameter of the elements it contains (sand, sludge, clay), their concentration, the viscosity of the suspension, the characteristics of the grit trap, etc.
The most frequently used circular grit traps are formed of a cylindrical-conical chamber with a shallow cone-shape with an upflow outlet pipe in the axis of their cylindrical part and a downflow outlet pipe at the tip of the cone.
The suspension to be treated is tangentially injected under pressure into the upper cyclindrical part. A tubular opening in the axis of the cylindrical part is used to evacuate the upflows. Along this same axis, an opening at the base called apex evacuates the material reaching the downflow outlet.
The supply of suspension to be treated is conducted tangentially under pressure so as to achieve a centrifugal force which may reach more than 1000 g. It causes the formation of a vortex made up of two flows: an outer descending flow which carries with it the largest and densest particles (coarse grains, water and a small quantity of fines) towards the downflow outlet, and a central rising flow which evacuates the major part of the xe2x80x9cfinesxe2x80x9d.
With this type of circular grit trap, the upflow is practically rid of the grains larger than a given size X, the so-called cut-off value, but a certain proportion of fines (smaller than X) is carried into the downflow. The downflow is therefore never completely free of fines.
In order to improve the performance of circular grit traps, several developments have been put forward:
One development consisted of increasing the cone angle of the body of the grit trap to 90xc2x0 or more. With this arrangement it is possible to increase grading, that is to say to enable two particles of close density and of different but close size to be better separated from one another.
Another development consisted of making an injection of water tangentially into the lower part of the body of a circular grit trap of shallow cone shape (less than 30xc2x0). With this technique, it is possible to improve the elimination of the fines carried with the down flow. The principle of said improvement consists of replacing the quantity of water+sludge suspension which should normally evacuate through the downflow outlet, by fresh water. The quantities replaced in this way are eliminated via the upflow outlet. To achieve this, several successive injections of water are made above the apex, just at the point where the descending flow is contiguous with the rising flow. They are made tangentially in,the direction of the outer vortex and enable the progressive replacement of the suspension so as not to impair the descending flow, which would lead to loss of sand via the upflow outlet. The sludge, replaced gradually, is repelled towards the inner rising flow and eliminated via the upflow outlet.
In water treatment methods by flocculation-decantation using ballasted floc, it is sought to have the lowest possible loss of sand. For this purpose, the circular grit traps used are chosen for the cut-off value they can offer, for a sand particle size that is smaller than that of the microsand used as ballast. The ballast microsand is therefore practically recycled without any loss at the downflow outlet, while the lightest fines distribute themselves more or less proportionally to the flow of water between the upflow and downflow, and the heaviest fines distribute themselves statistically according to their size relative to cut-off diameter of the circular grit trap.
For some applications, the recycling of fines at the down flow is not desired, either to avoid unwanted concentration of dense fines in the decanter, which could lead to fouling or sand loss, for example for the ballasted floc decantation treatment of a river in spate containing silt; or to minimize the risk of recycling any undesirable microorganisms such as Cryptosporidlumn or Giardia with the ballast sand.
Up until now, the methods of decantation-flocculation using ballasted floc (with microsand or any other type of recycled granular ballast) use xe2x80x9cconventionalxe2x80x9d circular grit traps whose cone angle is in the order of less than 30xc2x0, to separate the excess sludge at the upflow outlet and the granular ballast to be recycled.
In this type of circular grit trap, the fine particles forming the sludge are divided between the downflow and the upflow in a ratio approximately equal to the ratio of the water flows exiting through the same openings, that is to say usually in a ratio of approximately 10/90 to 30/70. The particles having a density close to the density of the sand tend to be recirculated in the down flow.
The purpose of the present invention is to provide better washing of the granular material that is recycled during water treatment operations using ballasted floc decantation. This objective may be sought in particular:
to treat water with a high fine content, such as water from rivers for example,
to minimize possible recycling of pathogenic organisms.
Another purpose of the invention is to enable the application of a specific treatment to the sand and the residue of the recycled sludge, taking advantage of the good contacting capabilities provided by this injection mode.
A further purpose of the invention is to put forward an innovative design for a circular grit trap with injection of water into the down flow which optimises the possibilities of sand washing.
These different objectives are achieved through the invention which concerns a water treatment method including at least one flocculation-decantation step with ballasted floc, using at least one granular material, and at least one treatment step with a circular grit trap to treat at least one part of the sludge derived from said flocculation-decantation step, for the purpose of recycling at least one part of said granular material contained in the sludge, characterized in that said recycling step is implemented in at least one circular grit trap in which an auxiliary liquid is injected tangentially into said sludge essentially in the same direction as the latter.
The invention therefore proposes using, for a flocculation-decantation method, a particular type of circular grit trap, namely a circular grit trap using a tangential injection of auxiliary liquid.
According to one preferred variant, this auxiliary liquid is injected into a cylindrical injection chamber provided downstream from and at the exit of the conical chamber of a circular grit trap. Said method may be implemented by means of a new design of circular grit trap which is described below.
Preferably, said auxiliary liquid is injected in said cylindrical injection chamber at an injection rate of between 50% and 200% of a value equal to the global flow rate of the downflow of the said circular grit trap less the flow of granular material to be recycled.
In best preferred manner, said auxiliary liquid is injected into said cylindrical injection chamber at an injection rate corresponding to the global flow rate of the downflow of the said circular grit trap less the flow rate of the granular material to be recycled.
The method of the invention may be applied using any type of granular material. However, this granular material is preferably a micro-sand having a diameter of between 40 and 400 xcexcm; preferably between 60 and 200 xcexcm.
According to one preferred variant, said auxiliary liquid is made up of water to which is optionally added, in continuous or periodic manner, at least one washing, disinfecting or ballasted flocculation promoter reagent. Said reagent may therefore advantageously be chosen from among the group made up of sodium hydroxide, hydrochloric, sulphuric, nitric, phosphoric, citric, formic, carbonic acids, bleach, dissolved chlorine, chlorine dioxide, gaseous or dissolved ozone, hydrogen peroxide, aluminium or iron salts, flocculation polymers.
The present invention also concerns a circular grit trap specially designed to be used for the above-described method, but which may also be used for any other application requiring the separation of a particle material from a suspension of the latter. This new circular grit trap may therefore be used at the tail end of a sand removal unit positioned either at the head of the plant or on the treatment network. In this configuration, the circular grit trap will bring an improvement in the separation of sand, derived from sand removing operations, from its polluting organic fines.
The circular grit trap of the invention has a cylindrical part of diameter D provided with at least one tangential supply of the suspension to be treated, and a conical part ending in an apex, and is characterized in that it is provided with a cylindrical chamber for the injection of an auxiliary liquid at the exit of said conical part and having a diameter Dxe2x80x2 of between 0.35 D and 0.65 D, said cylindrical chamber having at least one tangential supply of auxiliary liquid along the same feed direction as said tangential supply of the suspension to be treated.
Preferably, the diameter Dxe2x80x2 of said cylindrical chamber is between 0.4 D and 0.55 D.
In best preferred manner, the diameter d of said cylindrical chamber is 0.48 D.
Preferably, said conical part has an angle xcex1 of between 30 and 150xc2x0, advantageously between 55 and 80xc2x0.
In best preferred manner, angle a lies between 60 and 75xc2x0.
One innovative aspect of the invention, is therefore to use the injection of an auxiliary liquid into a cylindrical chamber connected to the end of the apex of a circular grit trap having a wide angle (greater than 30xc2x0) using a combination of characteristics that has never yet been described to the knowledge of the Applicant.
Such combination of characteristics enables careful evacuation of all the sludge via the upflow outlet of the circular grit trap, irrespective of its size, shape and density, while recovering practically all the granular material, that is as clean as possible, at the downflow outlet.
With the wide angle of the downflow outlet, it is possible to recover granular material of the desired particle size with a recovery rate of over 99%. By means of the narrow cut-off value obtained, the heavier sludge such as clay is eliminated better.
The other sludge is eliminated by the injection of auxiliary liquid into the injection chamber. Said mode of injecting auxiliary liquid has not ever been proposed up until now to the knowledge of the Applicant.
Using an injection rate such as defined above, the water containing fines surrounding the granular material arriving in the injection chamber is replaced by an equivalent flow of injected auxiliary liquid.
The unique combination of these three techniques: wide angle downflow outlet, injection of water, and use of a separate cylindrical injection chamber, makes it possible to recover between 90 to over 97% of sludge at the upflow outlet. By comparison, a conventional cleaning grit trap only eliminates 80 to 90% of sludge.
In addition, by separating the grit trap from the water injection chamber, it is possible to properly conduct the grading function in the grit trap with a wide cone angle, and the fines elimination function in the injection chamber in which the water arrives tangentially. A clean granular material is therefore obtained, that is more or less concentrated, which in no way hinders the envisaged applications.
Finally, it will be noted that the proposed circular grit trap facilitates sludge rise and minimizes the risk of fouling downflow outlets through excess heavy sludge in the granular materials.
According to another preferred aspect, the circular grit trap comprises quick connection means to a source of auxiliary liquid.